I've become a HUGE fan of my tablet as a flight instructor. It keeps me organized and has almost eliminated my need for a flight bag (still need to carry a few small things) without feeling like I'm leaving anything behind.
I use a 10" Android tablet, but I am finding it very useful on the ground and in the cockpit. Here are the things I'm doing with it. I'm sure all the equivalent functions are available on iPad.
I use an EFB app (Avilution AviationMaps). Of course this works as a navigational aid, but much more so as a training tool. By holding it up in the cockpit I can pinch zoom and show areas of the sectional chart they need to know, remind them where to find frequencies, help them make the connection between what they see outside and what is on the sectional chart for teaching pilotage. I use this on every flight to teach some application of using the charts.
I require my students to show up with a print-off of the METAR/TAF, Notams, and TFR map for each lesson. The first time I teach them to do this I just pull up the web sites on the tablet (it's 4GLTE so I do this on the ramp) and show them how to retrieve this information.
I've created a weight and balance spreadsheet with the envelopes and put it on my web site. I teach the students how to check weight and balance interactively on the tab and then they have the same tool available at home. Feel free to steal a copy and modify:
http://www.highcountryair.com/page/weight-and-balance-calculations
I have all the FAA handbooks and PTS in PDF so I quickly pull those up to show graphics of what a particular maneuver looks like, to help them visualize what I'm explaining. I really wish there was a PDF of ASA's Visualized Flight Maneuvers Handbooks, as I think they are great, I still keep a paper copy in my bag but might just end up scanning it and creating my own PDF for tablet use.
I've got copies of many different POH's in PDF, so I can also quickly pull those up to explain performance graphs and operating procedures.
I have downloaded the relevant regs from
http://www.gpo.gov/fdsys/pkg/CFR-2004-title14-vol1/content-detail.html and have these split up by major parts. Then I've notated the regs with bookmarks, highlighting/underlining, and notes in my PDF app.... so I can pull these up quickly and explain why we must do certain things.
I record almost all of my flights in Google Tracks, upload the tracks to Google Maps, and send the student a link to help illustrate what was right/wrong about a flight (particularly useful when doing ground reference maneuvers and traffic patterns).
Square/Paypal/whatever-payment-solutions-you-like are essential to getting paid and making book-keeping simple (if you're independent/self-employed).
I use a note taking app (FreeNote on Android) to jot down ATIS and miscellaneous notes in flight, and to sketch illustrations if needed to help visualize something I'm teaching.
I use Excel to keep training records for each student, one row for each flight, so I have a solid record of what we have covered.
I have student checklists in Excel for things like pre-solo requirements, so I can check those off as they are learned and as they demonstrate proficiency and show those to the students as we go so they can visualize their own progress, which helps keep them motivated.
For in-flight reference I put my lesson plan either in Excel or in my FreeNote application so I can be sure to follow the plan and not forget any important details.
I keep my student documents sync'd to the cloud with SugarSync, so I can review those on my PC and know I have a backup if the tablet blows up.
I keep my schedule sync'd on my tablet with Google Calendar so I can easily schedule additional lessons right from the ramp and not run into personal conflicts.
I find the 3G/4G Verizon month-to-month 1GB service indispensable. I can grab weather/nexrad, file flightplans, check email, process credit card payments, etc etc etc from the plane and ramp. I would feel very limited with just wifi at this point, and the $20/month for service is easy to swallow. I use a small fraction of the 1GB plan.